This invention is related to the field of processes that produce polymers that comprise ethylene.
There are many production processes that produce polymers that comprise ethylene. There are also many manufacturing processes that use these polymers to produce useful items. One of these manufacturing processes is called blow molding. In general, blow molding is useful for producing hollow plastic products. A principle advantage of blow molding is its ability to produce hollow shapes without having to join two or more separately molded parts.1 In order to produce a good quality blow molded product, one needs to start with a good quality polymer. However, producing such good quality polymers is difficult. Therefore, the inventors provide this invention so that such good quality polymers are more readily obtainable.
1 See SPI Plastic Engineering Handbook, Fifth Edition, pages 341-382, edited by M. L. Berins, published by Van Nostrand Reinhold (1991) 
It is an object of this invention to provide a process to polymerize ethylene with at least one olefin to produce a polymer.
It is also an object of this invention to provide a polymer.
In accordance with this invention a process is provided. This process comprises polymerizing ethylene with at least one olefin to produce a polymer, wherein said polymerizing is conducted in a polymerization zone, where said polymerization zone is essentially free of cocatalyst, and wherein said polymerizing is conducted using a catalyst, where said catalyst comprises chromium oxide on a support, and where said support comprises silica, and where said support is essentially free of titania, alumina, and phosphates, and where said catalyst has been activated, and where said catalyst after being activated is then reduced, and wherein said polymerizing is conducted in the presence of hydrogen, and wherein the majority of said olefin is produced by said catalyst, during said polymerizing.
In accordance with this invention, an embodiment is directed to a polymer comprising the following properties: a density from 0.94 to 0.96, a melt index from 0.01 to 0.1 g/10 min., a high load melt index from 5 to 100 g/10 min., and a heterogeneity index from 4 to 14, a shear ratio from 50 to 200, and a Bottle ESCR 33 percent higher than a comparison resin.
This invention, in general, is a process comprising polymerizing ethylene with at least one olefin to produce a polymer.
This polymerizing is conducted in a polymerization zone. This polymerization zone can be any convenient reactor. However, currently, it is preferred when the polymerization zone is a loop reactor. It is even more preferred when said polymerizing is conducted in a loop reactor under slurry polymerization conditions. Currently, the preferred diluent for slurry polymerization is isobutane. One of the important aspects of this polymerization zone is that it is essentially free of cocatalysts. This is because cocatalysts adversely affect the formation of the desired product. Examples of such cocatalysts are trihydrocarbylboron compositions and trihydrocarbylaluminum compositions. In general, the amount of these cocatalysts should be less than 1 part per million based on the weight of the diluent. However, it is most preferred when no cocatalyst is added to the polymerization zone.
The catalyst used in this invention comprises chromium oxide on a support. The amount of chromium on said support is in the range of about 0.01 to about 5 weight percent, preferably about 0.1 to about 3 weight percent, and most preferably from 0.5 to 2 weight percent, where such weight percents are based on the weight of the support. The catalyst should be essentially free of chromium halide compounds and organo-chromium compounds (hereafter xe2x80x9cHO compoundsxe2x80x9d) as these HO compounds can adversely affect the formation of the desired polymer. In general, the amount of these HO compounds should be less than 0.2 weight percent based on the weight of the support. However, it is most preferred if no HO compounds were added to the catalyst or to the polymerization zone.
The support comprises silica. Additionally, such support has silica as its major component by weight. It is most preferred when said support consists of silica, with little, if any, impurities. The support should be essentially free of titania, alumina, and phosphates (hereafter xe2x80x9cTAP compoundsxe2x80x9d) since these TAP compounds can adversely affect the formation of the desired product. In general, the amount of these TAP compounds should be less than 0.2 weight percent based on the weight of the support. However, it is most preferred when no TAP compounds are added to the catalyst or to the polymerization zone.
The support should have a surface area from about 200 to about 550 square meters per gram. It is more preferred when the support has a surface area from about 225 to 425 square meters per gram, and it is most preferred when said support has a surface area from 250 to 400 square meters per gram. Surface areas below about 200 m2/g have less activity, while surface areas above about 550 m2/g produces polymers that have a die swell that is too high, an amount of long chain branching that is too low, and possibly, a melt index that is too low.
The support should have a pore volume from about 0.7 to about 2.5 cubic centimeters per gram. It is more preferred when the support has a pore volume from about 0.8 to about 1.8 cm3/g, and it is most when said support has a pore volume from 1 to 1.7 cm3/gram. Pore volumes below about 0.7 cm3/g have less activity, while pore volumes above about 2.5 cm3/g adversely affect the formation of the desired polymer.
Methods of producing these types of catalysts are known in the art. See for example, U.S. Pat. Nos. 3,900,457; 4,081,407; 4,392,990; 4,405,501; 4,735,931; 4,981,831; the disclosures of which are hereby incorporated by reference.
The catalyst should be activated in an oxidizing ambient at a temperature from about 600 to about 900xc2x0 C. It is even more preferred when the temperature is from about 625 to about 850xc2x0 C., and it is most preferred when the temperature is from 645 to 820xc2x0 C. At temperatures below about 600xc2x0 C. the activity of the catalyst is reduced and the physical properties of the polymer are adversely affected. At temperatures above about 900xc2x0 C. the catalyst can begin to sinter, thus hurting the polymerizing properties of said catalyst. Currently, the preferred oxidizing ambient is air. This activation is carried out for a time period of about 1 minute to about 50 hours. This allows a portion of the chromium in a lower valance state to be converted to a hexavalent state.
After activation, the catalyst is then reduced in an reducing ambient at a temperature from about 200 to about 550xc2x0 C. It is even more preferred when the temperature is from about 225 to about 525xc2x0 C. and it is most preferred when the temperature is from 250 to 500xc2x0 C. At temperatures below about 200xc2x0 C. the catalyst may not be sufficiently reduced. At temperatures above about 550xc2x0 C. the activity of the catalyst can be adversely affected and the melt index of the polymer can be too low. Currently, preferred reducing ambients are a mixture of carbon monoxide and nitrogen, or just carbon monoxide. This reduction is carried out for a time period of about 1 minute to about 50 hours. This allows a portion of the hexavalent chromium to be converted to a lower valent state.
In general, the oxidizing ambient promotes the formation of hexavalent chromium while the reducing ambient promotes the formation of chromium atoms that have a valance less than 6.
Said polymerizing is conducted at a temperature from about 94 to about 112xc2x0 C. However, it is preferred when said polymerizing is conducted at a temperature from about 96 to about 110xc2x0 C. and it even more preferred when said polymerizing is conducted at a temperature from 98 to 108xc2x0 C. At temperatures below about 94xc2x0 C. the activity of the catalyst is adversely affected. At temperatures above about 112xc2x0 C. the reactor could foul due to the swelling of the polymer.
It is important to this invention that hydrogen be present in the polymerization zone. This is because hydrogen promotes the formation of the desired polymer by lowering the density of the polymer into the desired range. Currently, about 0.01 to 2 weight percent hydrogen can be used. However, it is preferred when about 0.05 to 1.5 weight percent is present in said polymerization zone, and it is more preferred when 0.1 to 1 weight percent of hydrogen is present in said polymerization zone. These weight percents are based on the weight of the diluent in said polymerization zone. It should be noted that hydrogen is often used to modify the molecular weight of a polymer, however, it is believed that this is the first time hydrogen has been used to produce a specific polymer of a specific density range. In other words. this is the first time hydrogen has been used to fine tune the density of a polymer.
The ethylene used should be polymerization grade ethylene. However, 100% ethylene produces better results.
The olefins are alpha-olefins having from 4 to 12 carbon atoms. One of the important features of this invention is that the catalyst produces these olefins during said polymerizing. This eliminates the need for having a source of alpha-olefins for use as comonomers. However, small amounts of alpha-olefins can be added, if desired, as long as the majority of said olefins, by weight, are generated in situ. Currently, 1-butene, 1-hexene, and 1-octene are the most preferred olefins.
The polymer produced needs to have the following properties in order to be a polymer that is good for blow molding applications.
The density needs to be from about 0.94 to 0.96 grams per cubic centimeter. However, it is preferred when the density is from about 0.945 to 0.96 g/cm3 and it is more preferred when the density is from 0.95 to 0.955 g/cm. Density is determine in accordance with ASTM D1505.
The melt index needs to be from about 0.01 to 1 grams per ten minutes. However, it is preferred when the melt index is from 0.1 to 0.8 g/10 min. and it is even more preferred when the melt index is from 0.2 to 0.6 g/10 min. Melt Index is determined in accordance with ASTM D 1238.
The high load melt index needs to be from about 5 to 100 grams per ten minutes. However, it is preferred when the high load melt index is from 10 to 80 g/10 min. and it is even more preferred when the melt index is from 20 to 70 g/10 min. High Load Melt Index is determined in accordance with ASTM D 1238.
The heterogeneity index (Mw/Mn) needs to be from about 4 to about 20. However, it is preferred when the heterogeneity index is from 6 to 20 and it is even more preferred when the heterogeneity index is from 8 to 18, and it is most preferred when the Heterogeneity index is from 10 to 16. The heterogeneity index was determined by gel permeation chromatography.
The Shear Ratio (HLMI/MI) needs to be from about 50 to about 200. However, it is preferred when the shear ratio is from about 65 to about 120 and it is even more preferred when the shear ratio is from 70 to 100.
The Bottle ESCR of an inventive resin should be about 33 percent higher than a comparison resin. However, it is more preferred when the Bottle ESCR of an inventive resin is 50 percent higher, and it is most preferred if the Bottle ESCR is 100 percent higher. For the purposes of this application the comparison resin is a resin made with the same catalyst as an inventive resin except that the catalyst that makes the control resin has not been reduced. Additionally, these resins are produced under substantially, if not identical, polymerization conditions.
Polymerization can be carried out in any manner known in the art such as gas phase, solution or slurry polymerization conditions. A stirred reactor can be utilized for a batch process, or the reaction can be carried out continuously in a loop reactor.
A preferred polymerization technique is that which is referred to as a particle form, or slurry process, wherein the temperature is kept below the temperature at which polymer swells badly. Such polymerization techniques are well known in the art and are disclosed, for instance, in Norwood, U.S. Pat. No. 3,248,179, the disclosure of which is hereby incorporated by reference.
Two preferred polymerization methods for the slurry process are those employing a loop reactor of the type disclosed in Norwood and those utilizing a plurality of stirred reactors either in series, parallel or combinations thereof wherein the reaction conditions are different in the different reactors.
The following examples are provided to further inform a person of ordinary skill in the art.